The present invention concerns an apparatus for measuring the vector voltage ratio of two a.c. signals having different phase and amplitude. In prior art methods for measuring the vector voltage ratio of two input signals, the conventional method involved detecting by synchronous rectification the in-phase component and the orthogonal component of a second input signal relative to a first input signal. In these methods the in-phase component is determined by employing the first input signal to synchronously rectify the second input signal. The orthogonal component is determined by employing the first input signal shifted in phase by .pi./2 to synchronously rectify the second input signal.
When precisely measuring the vector voltage ratio by the above method, it is necessary to eliminate the total phase error of the input signals at the synchronous rectifier including the phase error due to the signal paths. If such phase errors are not eliminated, the in-phase component and orthogonal component are not precisely detected. In order to eliminate the effect of phase error of the synchronous rectifier, an apparatus for measurement of the vector voltage ratio combining a synchronous rectifying circuit (e.g. a modulator) and an analog circuit is shown in Japanese patent number SHOWA 53-26823(KOUKOKU) entitled "Apparatus for Measurement of Vector Voltage Ratio". In order to eliminate the offset phase error which is inherent in the synchronous rectifying circuit as well as the phase error due to the two signal paths to the synchronous rectifying circuit, the synchronous rectifying circuit itself is improved. (See, for example, Japanese UTILITY MODEL Appl. No. SHOWA 50-79922 entitled "Apparatus for Compensation of Phase Error of Phase Detector".) But these conventional techniques require complicated analog techniques.